Bell began practising medicine in Glasgow in 1868.^[4] In 1870, he was elected a Fellow of the Faculty of Physicians and Surgeons, Glasgow. He was also a Licentiate of the Royal College of Surgeons, Edinburgh.^[2][3]

In 1876, Bell founded the Glasgow Hospital for Diseases Peculiar to Women, later the Glasgow Hospital for Women.^[2][5] He worked there for 21 years as a senior physician.^[5]

Vance Ferrell wrote that, in the 1870s, Bell developed a treatment for diphtheria and a method for treating smallpox that was intended to prevent secondary fever. Ferrell also wrote that, by the 1880s, Bell had linked constipation with illness and used the term "autotoxemia" for the absorption of toxins into the bloodstream.^[6]

Bell moved to London in 1904.^[7]

In 1909, Bell declined an offer of a baronetcy from King Edward VII, who had read one of Bell's books on cancer and had an interest in the subject.^[8] In the same year, Bell gave a speech on the benefits of a fruitarian diet at the newly opened international headquarters of the Order of the Golden Age in London.^[9] He was also a council member of the Order and vice-president of the International Cancer Research Society.^[4][10]

From 1910, Bell led cancer research at Battersea Anti-Vivisection Hospital.^[8] He used the hospital to promote his view that surgical treatment for cancer was unnecessary and that cancer could be prevented through dietetic and hygienic measures.^[7][11] He recommended fresh air and a vegetarian diet of uncooked vegetables and fruit, nuts, and dairy products to his cancer patients.^[5][12]

In 1912, the British Medical Journal published an article titled "Cancer, Credulity, and Quackery", which accused Bell of promoting pseudoscience. Bell sued the journal for libel and was awarded £2000 in damages.^[8]

In 1923, Bell was charged with breaching medical etiquette. The allegation was that he had prescribed treatment for and attended a woman with cancer without having seen her in person. He was cleared of the charges.^[3]

Bell made early micrographs using a camera that he built himself. One micrograph, taken in 1872 before dry plates were available, required an exposure time of at least three-quarters of an hour.^[8]

In 1893, Bell published a collection of poetry titled A Physician's Poems. David Herschell Edwards later included a biographical notice and a selection of Bell's poems in One Hundred Modern Scottish Poets.^[2]

Bell published his autobiography, Reminiscences of an Old Physician, in 1924.^[13] It included a selection of his micrographs.^{[13]: xi–xii}